This invention relates to a method of testing the proper functioning of an anti-lock brake control system which is incorporated with a motor vehicle hydraulic brake system for automatically controlling the degree of rotational anti-lock during braking.
Hydraulic brake systems of motor vehicles are usually equipped with some form of anti-lock brake control system, referred to as the antiskid system in common parlance, in order to prevent wheel lockup during braking. Should the brakes lock the vehicle wheels, they would begin to skid. Generally, in hydraulic brake systems, brake fluid is sent from a master cylinder to wheel cylinders at respective vehicle wheels upon application of a brake pedal. The wheel cylinders act to slow down or stop the wheel speed which in turn slow or stop the vehicle. The anti-lock brake control system includes a set of wheel speed sensors, one for each vehicle wheel. The control system automatically decreases, holds or increases the fluid pressures in the wheel cylinders as dictated by the varying speeds of the vehicle wheels.
Several methods have been suggested for testing, or detecting or diagnosing troubles in, hydraulic anti-lock brake control systems. Japanese Patent Laid-Open Publication No. 59-184052 represents one such known method, which is for a system that controls brake fluid in response to the lowest of sensed wheel speeds. This prior art method detects fluid pressure oscillations taking place within the wheel cylinders due to the operation of the anti-lock brake control system in response to quasi wheel speed signals, with the vehicle held at a standstill. The method makes possible the confirmation of the fact that the control system remains inactive during normal braking, but that the control system positively operates as required in response to the lowest of the wheel speeds.
An objection to this prior art is that it necessitates the provision of means for sensing the fluid pressure oscillations within the wheel cylinders. An additional objection arises in the case where the test equipment is constructed as an independent, portable unit. In that case the pressure oscillation sensors attached to the wheel cylinders must be electrically connected to the pilot lamp circuit included in the separate test equipment.
Japanese Patent Laid-Open Publication No. 61-36008 proposes an anti lock brake control system with a self-diagnostic capability. A switch for starting the self-diagnosis of the system is disposed within easy access from the vehicle driver. A simple actuation of the start switch does not actually start the self-diagnosis, however, unless some additional input conditions are met. Thus the control system is protected against any accidental commencement of the procedure during vehicle travel even though the start switch itself is capable of ready actuation by the driver.
However, this prior art system is also unsatisfactory in that it permits self-diagnosis of only the arithmetic and logic operations of the electronic control circuitry. The system is incapable of detecting troubles in the actual function of the whole control system.